The present invention, in some embodiments thereof, relates to microRNAs, more particularly, but not exclusively, to expression or repression of same in pancreatic beta cells for modulation of insulin levels.
In adult beta cells, insulin transcription is tightly regulated by a network of transcriptional activators and repressors. A few transcriptional repressors of the Insulin gene have been characterized, including, NR×2.2, Insm1 (Insulinoma-Associated 1/IA1), Sox6, Bhlhe22 and Crem. Transcription activators have also been characterized including Pdx1, MafA, NeuroD, Pax6 and Isl1 which maintain the beta cell fate and activate insulin transcription in response to elevation in plasma glucose. Thus, a fine balance between these opposing transcription factors must be kept for effective insulin synthesis.
The recently discovered microRNA (miRNA) family of posttranscriptional regulators, provide an additional regulatory layer that may play an important role in the adult endocrine pancreas. miRNAs are important for beta cell differentiation and specific miRNAs have been proposed to regulate beta cell genes. For example, miR-124a was shown to affect the expression of FoxA2 and Pdx1, the secretory pathway proteins, SNAP25 and Rab3a, as well as the ion channels Kir6.2 and Sur1 [Baroukh, N. et al. (2007) J Biol Chem 282, 19575-19588; El Ouaamari A. et al. (2008) Diabetes 57, 2708-2717].
The involvement of miRNAs in insulin secretion has also been contemplated. Poy et al. have shown that miR-375 affects insulin secretion through regulation of myotrophin expression, specifically, they showed that over-expression of miR-375 suppressed glucose-induced insulin secretion, and conversely, inhibition of endogenous miR-375 function enhanced insulin secretion [Poy M. N. et al. (2004) Nature 432, 226-230]. Xia et al. have shown that over-expression of miR375 reduces glucose-induced insulin secretion by down-regulating the expression of myotrophin in Nit-1 cells [Xia et al., Mol Biol Rep. (2010) Epub ahead of print]. Roggli E. et al. have shown an increase in miR-21, miR-34a and miR-146a in islets of NOD mice during development of pre-diabetic insulitis. According to their teachings, overexpression of miR-21 or miR-146a did not significantly affect insulin content, insulin promoter activity or pro-insulin mRNA levels, while overexpression of miR-34a led to a decrease in insulin content and insulin promoter activity accompanied by a reduction in pro-insulin mRNA level [Roggli E. et al., Diabetes (2010) 59(4):978-86].
To date, the only knockout model for an islet-enriched miRNA is the mouse knockout for miR-375. Genetic loss of miR-375 causes decreased beta cell mass due to impaired proliferation. Additionally, miR-375 mutants have increased alpha cell numbers, increased plasma level of glucagon and increased gluconeogenesis in the liver [Poy M. N. et al. (2009) Proc Natl Acad Sci USA]. Thus miR-375 provides an intriguing endocrine phenotype that encourages further evaluation of the role of miRNAs in vivo.
miRNAs are subject to extensive processing, including digestion by Drosha in the nucleus and by Dicer1 in the cytoplasm. Deletion of Dicer1 in the early pancreatic lineage, using a Pdx1-Cre mouse line resulted in inactivation of the entire miRNA pathway in the early pancreatic bud. This early inactivation of Dicer 1 causes pancreas agenesis, suggesting that miRNA are indeed important for pancreas organogenesis.
U.S. Application No. US 2009/0131348 describes methods and compositions of identifying a miRNA expression profile for a medical condition, such as pancreatic disease, and subsequently disclose methods for diagnosing and treating such as condition, by for example, downregulation of miRNA or by administration of synthetic miRNA molecules.
U.S. Application No. US 2005/227934 describes pancreatic islet microRNAs and methods for inhibiting same. Specifically, U.S. Application No. US 2005/227934 teaches anti-pancreatic islet microRNA molecules which are capable of inhibiting pancreatic islet microRNAs and use of same for treating diabetes.